1. Field of the Invention
The present invention relates generally to a device for determining the location and distance to a conductive conduit, and more particularly, it relates to an internally self-calibrating apparatus for determining highly accurate depth measurements of a concealed conductor, such as an underground pipe or cable, which is radiating an electromagnetic signal.
2. General Description of Prior Art
There are a number of devices available to determine the location and depth of electrically conductive structures, the most apparent application of such device being its use in finding underground cables and pipes. Throughout the evolution of these electronic devices, much attention has been given to the simplification of the design and user operability. Thus, more and more emphasis has been shifted to designing these sensory devices with internalized functions, obviating the need for user interface, and resulting in increased overall accuracy in measurements performed by the instrument.
Most electromagnetic-sensor depth-finding devices make use of receiving coils or antenna positioned so that their axes are parallel to the horizontal plane of the ground. These coils or antenna are sensitive to electromagnetic fields which can be made to radiate from a concealed source by direct application of an alternating current to the source, or by inductive coupling of this current with the source. Using the difference in field strength sensed between two vertically displaced receptors, the sensing devices are capable of determining the depth of the radiating source.
A good example of an earlier sensing device is described in the November 1965 issue of the BELL LABORATORIES RECORD. To calculate the depth measurements by use of this device, the operator first activates a lower receiving coil or antenna. The signal received by this receptor is amplified to a useful output level, and then a read-out meter is manually adjusted to use this output level as representative of a full scale signal response. Next, the lower receptor is disconnected, and the upper coil or antenna activated. The signal received from this receptor is amplified and sent to the previously adjusted meter, which is calibrated to reflect the ratio of the signals from the two receptors as a user-readable depth measurement. Although this device is effective when used properly, there is significant room for user error due to the necessity of manual adjustments and meter readings. Additionally, periodic calibration of the instrument is necessary to insure that the highly sensitive receiving coils are performing accurately.
Another device, described in U.S. Pat. No. 4,387,340 and related U.S. Pat. No. 4,520,317, significantly alleviated the possibility for user error by internalizing the sensor functions. This device uses a similar upper and lower coil arrangement as that used in earlier sensing devices, however, this device also employs two additional receiving coils. The additional coils are placed laterally apart, and equidistant from and perpendicular to a vertical line extending between the upper and lower coils. These laterally placed coils are used to determine the exact horizontal location of the radiating source, and thus increase the accuracy of any subsequent depth measurements. A sequencer controller was also introduced in this device which insures the automatic process of measuring the signal received by the bottom sensor, storing it in memory, holding the value of the gain for the amplification means, and amplifying the signal received by the top sensor using this stored value of gain. A computing means then calculates the depth of the radiating object based upon the amplified output voltages from the upper and lower receiving coils. This depth calculation is then displayed by a user-readable display means.
Although this device eliminates many of the manual steps that were once necessary, the circuitry involved is still very sensitive and complex. In all electromagnetic field sensing devices, very precise measurements must be made with the receiving coils. These measurements necessarily depend upon the relative accuracy and stability of the receiving coils and associated circuitry. In earlier devices, it was necessary to sacrifice sensitivity to very small signals in order to improve relative accuracy and stability. Additionally, the earlier devices operated in an open-loop fashion, necessitating periodic manual re-calibration, to assure that the receiving coils were providing accurate field readings. This allowed for error in measurements performed in between this re-calibration, and the inconvenience of periodic manual system adjustments.